This invention relates to means for detecting stall in a gas turbine engine and more particularly to a stall detection system that utilizes the existing engine fuel control and its computed parameters.
As is well known, stall is a phenomenon that may occur in the compressor of a gas turbine engine which, if allowed to persist unabated, would impair engine performance and/or lead to the destruction of the engine. While the theory of stall is not completely understood, suffice it to say that stall is that effect occasioned when a sufficient number of compressor blades stall and a momentary reversing of the airflow occurs through the compressor. This causes compressor discharge pressure to drop very rapidly and sometimes results in continual pressure oscillations until some corrective action is taken.
The art has seen a number of methods intended to either sense when stall is imminent and warn the pilot so that he can take corrective action or design the engine controls such that the area of engine operation where stall is likely to occur is avoided.
For example, fuel controls limit the amount of fuel admitted to the engine during acceleration so as to accelerate along a predetermined acceleration schedule that accounts for stall. Another method, which may be contemporaneously employed with this acceleration scheduling system is to measure compressor discharge pressure and open compressor bleed valves whenever a predetermined compressor pressure change or rate of change occurs. And still another method which is described in U.S. Pat. No. 3,867,717 and granted to John Theodore Moehring and Vigil Willis Lawson on Feb. 18, 1975 is the utilization of computed compressor pressures and turbine or exit temperatures as a means for calculating when stall is imminent.
While such stall detection and prevention means as described above may be effective for certain engines and/or their applications they are not always effective for other engines and/or their applications. For example, it may happen that under the same values of the computed compressor pressures or their rates and turbine temperatures or their rates another engine operation may occur which would lead to a false indication of stall; or the monitoring of the parameter may not be readily accessible or the inclusion of the sensing probes may interfere with the gas path and impair engine performance. Therefore the selection of the stall controller comes down to what stall system is best for that engine and its applications, what parameters are readily accessible, which system will provide the highest degree of accuracy, which one is fastest and a host of other considerations.
This invention contemplates monitoring the accelerating schedule which is already in existence and manifested by the fuel control and another engine parameter, namely compressor speed, burner or compressor discharge pressure or their rate of change. In essence, this system takes into account whether or not the fuel control is on the acceleration schedule and whether or not the compressor speed (RPM) is decreasing or not increasing. (In a twin spool engine configuration N may be either the low compressor speed (N.sub.1) or the high compressor speed (N.sub.2) ). If this occurrence takes place, then, in certain engines stall would ensue. Alternatively, this invention contemplates monitoring the acceleration schedule and P.sub.b (burner pressure) or P.sub.S3 (static compressor discharge pressure together with compressor inlet pressure (P.sub.2) to determine if stall is imminent. In this instance a computer produces an output signal that is proportional to the ratio of P.sub.3 to P.sub.2 or P.sub.B to P.sub.2.
Alternatively, this invention contemplates monitoring P.sub.B or P.sub.3 and the acceleration schedule produced by the fuel control and provide a stall warning signal whenever the fuel control is on or in proximity to the acceleration schedule, say up to 3 ratio units away in a W.sub.f /P.sub.b type of control, and the decay rate of P.sub.3 or P.sub.B is exceeding a predetermined value.
In any stall control system that senses when stall is imminent the computed stall signal can either be utilized to provide a warning signal to the aircraft pilot, as by a visual or sound signal so that he can take corrective action as by retarding the power lever or the signal can be utilized to initiate corrective action in one of the following ways.
1. de-rich engine fuel flow PA1 2. shutoff fuel PA1 3. open compressor bleeds PA1 4. change compressor stator vane angle PA1 5. change aircraft inlet geometry PA1 6. change engine outlet geometry
Furthermore, the signal can be incorporated in a system that would initiate an automatic stall recovery sequence by shutting-off fuel, start ignition and reinitiate fuel flow in a timed sequence.